In a discharge lamp wherein the arc discharge is initiated and/or maintained by means of a time varying drive signal such as a radio frequency (RF) current signal, there is a risk that under certain conditions, such arc discharge could become unstable and wander around the arc tube rather than remaining in a stable position as desired. This arc wandering or instability can cause the light output to flicker as well as to eventually extinguish altogether thus resulting in the need to restrike the lamp. This condition can occur in several different types of discharge lamps, including high intensity discharge (HID) and microwave excited sulfur lamps regardless of whether the lamp is electrodeless or electroded or, if in an electrodeless lamp, whether the drive signal is inductively or capacitively coupled to the fill within the arc tube. An example of one lamp which is susceptible to arc instability under certain conditions is an inductively-coupled, electrodeless HID lamp typically comprising an arc tube of quartz or other high-temperature light transmissive material in which an electrodeless toroidal arc discharge is developed. U.S. Pat. No. 4,810,938 issued to Johnson et al and assigned to the same assignee as the present invention illustrates the electrodeless HID lamp which utilizes an excitation coil disposed in surrounding relation to a portion of the arc tube and which is energized by an RF signal to develop a solenoidal field within the arc tube; the solenoidal field is effective for generating the above-described toroidal arc discharge. Thereafter, the excitation coil acts as a primary winding of a transformer, and the toroidal arc discharge, inductively coupled to the primary winding, acts as the secondary winding of this transformer. Other examples of further developments relating to this type of discharge lamp can be found in U.S. Pat. Nos. 4,812,702 issued to Anderson, 5,047,692 issued to Borowiec et al, 5,140,227 issued to Dakin et al and 5,150,015 issued to Heindl et al, all of which are assigned to the same assignee as the present invention and all of which are herein incorporated by reference. Another example of a lamp which may be susceptible to arc instability and which could be improved in performance by use of the present invention can be found in U.S. Pat. No. 4,975,625 issued to Lynch et al for an Electrodeless Lamp energized by microwave excitation.
We have found that when the excitation coil or other type of drive mechanism for exciting an arc discharge within an arc tube, is energized by RF current having a frequency greater than about 1 kilohertz, with one exemplary frequency being about 13.56 MHz, the discharge within the arc tube sometimes exhibits the undesirable instability previously discussed wherein the discharge can become constricted, wander in shape and position, and is prone to self-extinction. This is in marked contrast to the steady, fluffy appearance of the discharge under ideal stable conditions, during which time the discharge expands to occupy a substantial portion of the space within the arc tube and remain in close proximity to the arc tube wall. For the example of the electrodeless high intensity discharge lamp of U.S. Pat. No. 4,810,938 previously discussed, the stable toroidal discharge arc expands in a radial direction to fill the arc tube. In a lamp using a gas fill which includes a metal halide and an inert gas, it has been observed that when the arc tube is operated at a power level above some threshold value, such instability occurs as a result of the arc tube containing halogen in excess of metal halide stoichiometry. The excess halogen, often excess iodine, usually results from the loss of active metal from the arc tube dose. Operation of the other types of discharge lamps previously discussed at a power level in excess of some threshold value is also expected to result in the occurrence of the previously described unstable operation.
Therefore, it would be advantageous if an arc stabilization arrangement for arc discharge lamps could be provided that would allow for the operation of such lamps at power levels above those previously achieved without the risk of such arc discharge becoming unstable. It would also be advantageous if our invention could provide such improvement in arc stability without adversely impacting on the luminous efficacy of the lamp over the broad power range beneath the threshold level.
Another concern with the operation of an arc discharge lamp at an optimum power level and in a practical application such as in a fixture whereby the lamp can be oriented in any number of different manners such that the arc tube is inverted or has its axis tilted at any angle between vertical and horizontal, is that the arc stabilization arrangement remain equally effective as if the lamp were oriented in an upright manner. It would be advantageous if the arc stabilizing arrangement of the present invention could be equally effective with the arc tube oriented in a non-upright manner as it were for an upright orientation.
In carrying out the efforts which resulted in the present invention, it was discovered that the operation of the drive signal in conjunction with a frequency in the acoustic resonance band could provide the desired arc stability. Up until recently, the operation of a lamp at any frequency in the acoustic resonance band was considered to be very undesirable, that such operation itself was inherently unstable. Recent developments however have indicated that there are conditions under which acoustic resonance operation may in fact be beneficial. In U.S. Pat. Nos. 4,983,889 and 5,121,034 issued to Roberts and Allen et al respectively, both of which are assigned to the same assignee as the present invention, acoustic resonance operation is described for use with electroded metal halide lamps. Though effective for arc stabilization to the extent of arc straightening and the thorough mix of the gas fill constituents, such patents teach that the lamp should be driven at the acoustic resonance frequency and do not provide guidance for operation at higher frequencies as are utilized for electrodeless lamps. U.S. patent application Ser. No. 07/897,601 filed in the name of Dakin et al on Jun. 10, 1992 and also assigned to the same assignee as the present invention also discusses acoustic resonance operation of a metal halide lamp. This application of acoustic resonance operation of a metal halide lamp is effective for substantially reducing color separation in the principal arc discharge region, however, this application also teaches that the lamp should be operated using this acoustic resonance frequency to achieve the stated advantage. Recognizing that such acoustic resonance frequency typically occurs on the kilohertz range which is suitable for operating an electroded discharge lamp, electrodeless discharge lamps on the other hand are typically operated at frequencies approaching or exceeding the megahertz region and therefore could not benefit from these teachings. Accordingly, it would be advantageous to provide an acoustic resonance arc stabilization arrangement which would be equally effective for operation with electroded as well as electrodeless discharge lamps using the same stabilization techniques and which would be operable over a significantly wide range of operating frequencies used to drive the discharge.